This invention relates to an optical recording method and an optical recording device in which information is recorded as lengths of marks and spaces at high density by irradiating optical beams onto a recording thin film formed on a substrate.
Recently, optical recording media capable of recording, reading and erasing information have been commercialized. Furthermore, high-density rewritable optical recording media capable of recording qualified animation have been actively researched and developed.
Well-known rewritable optical recording media include phase-change optical recording media with recording layers either of chalcogenide thin films or semimetal thin films on a disc-shape substrate. The chalcogenide thin films comprise Te or Se, for example, Gexe2x80x94Sbxe2x80x94Te, Inxe2x80x94Se, or the like, as a base. The semimetal thin films comprise Inxe2x80x94Sb or the like. Magneto-optical recording media having metal thin films such as Fexe2x80x94Tbxe2x80x94Co as their recording layers also are well known. Further, there are also write-once-type optical recording media using pigment materials.
In phase-change optical recording media, recording thin films comprising the above-mentioned phase-change materials are instantly irradiated with laser beams focused on submicron-order size optical spots to heat the irradiated parts partially. When the temperature of the irradiated portion becomes equal to or higher than the crystalline temperature, the irradiated portion is changed to the crystalline state. When the irradiated portion is melted at a temperature higher than its melting point and is quenched, its state is changed to the amorphous state. Once either the crystalline state or the amorphous state is determined so as to correspond to the recording state, and the other to the erasing state (unrecorded state), information can be recorded reversibly by forming a pattern changing between the amorphous state and the crystalline state corresponding to information signals. Since the crystalline state and the amorphous state are different from each other in their optical characteristics, recorded signals can be read by optically detecting such different characteristics as a reflectivity change or a transmittance change.
In a magneto-optical recording medium, focused laser beams are irradiated on a magneto-optical recording thin film, so that the irradiated portions are partially heated. A magnetic field is applied to the heated portion in order to reverse the magnetizing direction of the magneto-optical recording thin film at the irradiated portions corresponding to the information to be recorded, thus recording information.
Methods of recording data on optical recording media at a high density include a mark-length recording. In the mark-length recording, marks with various lengths are recorded at various intervals (spaces), and the recording information is allocated to both the mark lengths and the space lengths. For example, in a phase-change recording medium, information can be recorded by setting amorphous regions as marks and crystalline regions as spaces.
In order to record information at higher density, it is necessary to shorten the mark lengths and the space lengths to be recorded. However, when the space lengths become shorter, the heat of a recorded mark-end affects the temperature increase at the starting-end of a mark to be recorded next. The position shift of a recorded-mark front-edge from the proper position caused by such thermal interference contributes to the aggravation of BER (bit error rate) when reading the information.
Examples of the method for improving the problem mentioned above are described, for example, in Unexamined Japanese Patent Application Tokkai Hei 5-234079 and Tokkai Hei 7-129959. In the applications mentioned above, a method of recording information by delaying the starting-end position of a recording pulse beforehand so as to compensate for the shift quantity of a mark-front-end position due to thermal interference is proposed. This recording method will be explained with reference to FIGS. 9(a)-(e).
FIG. 9(a) shows a pulse shape of data to be recorded. Levels of logic 1 and logic 0 correspond to marks and spaces, respectively. In accordance with the data shown in FIG. 9(a), the recording pulse shown in FIG. 9(b) is generated, thus forming the recording marks shown in FIG. 9(c) on an optical recording medium.
As shown in FIG. 9(c), when the recording density increases and therefore a space length between marks becomes shorter, the heat of a recorded mark 90 affects the front-edge of a mark 91 to be recorded. Consequently, the front-edge temperature becomes higher than that when the space length is sufficiently long. As a result, the front-edge of the mark 91 expands greatly as shown with a reference number 92, and therefore the front-edge is formed ahead of the proper position.
When a space (a period at level 0) in the data to be recorded is short, the correction for delaying the front-edge of the recording pulse so as to compensate for the delay quantity 93 is made as shown in FIG. 9(d). By this correction, the front-edge of the mark 91 is formed at the proper position corresponding to the front-edge of the recording data.
At a further higher density, not only a space length directly in front of a mark to be recorded but also a mark length in front of the space and a space length and a mark length further ahead, cause the temperature increase at the front-edge of the mark to be recorded, thus further affecting the front-edge shift quantity. Especially, the mark length in front of the space directly in front of the mark to be recorded affects greatly. This is because a long mark length requires a correspondingly long heating time by laser beams, and therefore the larger amount of heat is conducted to the front-edge of a mark to be recorded next. Thus, it is preferable that the delay quantity of the front-edge of a recording pulse is determined based on the lengths of a space directly in front of the mark to be recorded and of a mark in front of the space. When determining the delay quantity considering the lengths of a mark and a space further ahead, the front-edge shift can be corrected more precisely. Considering recording density, allowable error rate, processor capacity used in a device, cost, and the like, it will be decided how many spaces and marks before the mark to be recorded should be taken into account.
As described above, conventionally, a recording method in which front-edge shift is corrected considering the relation between a space length and a mark length directly in front of a recording mark and the front-edge shift of the recording mark is suggested. However, the position-shifts of not only the front-edge but also the end-edge of the mark cause the deterioration in the error rate when reading signals. It was found that the end-edge position of the recording mark was affected not only by space and mark lengths in front of the recording mark but also by space and mark lengths behind the recording mark. It can be conceived that this is because a cooling process of the recording mark is affected by the heat of the mark to be recorded next.
An object of the present invention is to provide an optical recording-reading method and an optical recording-reading device in which the deterioration in the error rate when reading signals is restrained by correcting the front- and end-edge shifts of recording marks caused by heat interference, thus obtaining high-quality reading signals.
In an optical recording-reading method of the present invention, when recording information on a recording thin film as lengths of marks and spaces, the position of a mark-end at the end of recording is changed according to the lengths of a mark to be recorded, of a space directly behind the recording mark and of a mark behind the space. According to this recording method, the edge shift of the mark-end caused by heat interference is corrected, thus preventing the error rate when reading signals from deteriorating.
The position of a mark-end at the end of recording may be changed according not only to the lengths of the space directly behind the recording mark and of the mark behind the space but also the lengths of at least one space and one mark further behind. In combination with this, the recording-start position of the starting-end of the mark may be changed according to the length of the mark to be recorded and a space length directly in front of the mark (and the length of a mark in front of the space). Further, by modulating the irradiation power of optical beams between a recording level and an erasing level lower than the recording level, the power of the optical beams irradiated to a portion behind a mark is made lower than the erasing level for a predetermined period. It is also preferable to combine this method. It is effective to restrain the heat interference between marks.
In another optical recording-reading method according to the present invention, the optical beam power corresponding to the starting-end and the end of a recording mark is changed. For example, the optical beam power is reduced so as to compensate for the increase in temperature due to heat interference. Specifically, when irradiating the optical beams to a recording thin film, the power of the optical beam irradiated to the starting-end end is changed according to the length of a recording mark and the length of a space directly in front of the mark (and the length of a mark in front of the space). The power of the optical beams mentioned above is modulated in the pulse train including the starting-end, the intermediate portion, and the end according to the lengths of marks and spaces. Then, the power of the optical beam irradiated to the end is changed according to the length of a mark to be recorded and the length of a space directly behind the mark (and the length of a mark behind the space).
Also in this case, the optical beam power corresponding to the starting-end or the end of a recording mark may be changed according to not only the lengths of a space and a mark directly in front of and behind the recording mark but also the lengths of at least one space and one mark further ahead and behind. In addition, it is also preferable to combine the method of lowering the power of the optical beam irradiated to a portion behind the mark for a predetermined period compared to the erasing level by modulating the irradiation power of the optical beam between the recording level and the erasing level lower than the recording level.
A first configuration of an optical recording-reading device according to the present invention to realize the optical recording-reading method mentioned above comprises: a basic pulse generator that generates a starting-end pulse corresponding to the starting-end of a mark, an end pulse corresponding to the end of the mark, and at least one intermediate pulse corresponding to the intermediate portion of the mark; a recording mark detector that detects the length of a recording mark; a back space detector that detects the length of a space directly behind the recording mark; a back mark detector that detects the length of a mark behind the space directly behind the recording mark; an end pulse delaying circuit that generates a delayed end pulse that is obtained by delaying the end pulse so as to compensate for the delay quantity determined from the output signals from the recording mark detector, the back space detector, and the back mark detector; a pulse synthesizer that generates a recording pulse that is obtained by synthesizing the starting-end pulse, the intermediate pulse, and the delayed end pulse; and a laser driver that modulates the optical beam power based on the recording pulse.
A second configuration of an optical recording-reading device according to the present invention comprises: a basic pulse generator that generates a starting-end pulse corresponding to the starting-end of a mark, an end pulse corresponding to the end of the mark, and at least one intermediate pulse corresponding to the intermediate portion of the mark; a recording mark detector that detects the length of a recording mark; a front space detector that detects the length of a space directly in front of the recording mark; a starting-end power setting circuit that sets the optical beam power corresponding to the starting-end pulse based on output signals from the recording mark detector and the front space detector; and a laser driver that modulates the optical beam power based on output signals from the basic pulse generator and the starting-end power setting circuit.
A third configuration of an optical recording-reading device according to the present invention comprises: a basic pulse generator that generates a starting-end pulse corresponding to the starting-end of a mark, an end pulse corresponding to the end of the mark, and at least one intermediate pulse corresponding to the intermediate portion of the mark; a recording mark detector that detects the length of the recording mark; a back space detector that detects the length of a space directly behind the recording mark; an end power setting circuit that sets the optical beam power corresponding to the end pulse based on output signals from the recording mark detector and the back space detector; and a laser driver that modulates the optical beam power based on output signals from the basic pulse generator and the end power setting circuit.